Micro-encapsulation is a process in which tiny particles or droplets are surrounded by a coating to give small capsules that have many useful properties. In a relatively simplistic form, a microcapsule is a small particle or droplet with a uniform shell around it. The small particle or droplet inside the microcapsule is referred to as the core, internal phase, or fill, whereas the shell is sometimes called a coating, a wall or membrane.
The reasons for microencapsulation are countless. Examples of possible uses of microencapsulation include:                prevention of adverse interactions between the core material of the microcapsules and the environment of these microcapsules (atmosphere, product matrix, stomach etc.)        improvement of the handling properties of the core material (e.g. by improving dispersibility or reducing stickiness)        manipulation of the rate at which the core material can participate in dynamic processes (e.g. controlled release of drugs or pesticides)        masking of the taste or odour of the core material.        
A huge variety of materials has been used as coating agents in the preparation of microcapsules. These materials include hydrophilic coating agents such as maltodextrin, alginate, gelatine, whey protein, hydroxy propyl methylcellulose, sucrose etc. Hydrophilic coating agents are particularly useful in case the microcapsules need to quickly release the core material when brought into contact with water. For some applications, however, it is important that the core material is not released when the microcapsules come into contact with water or that is only released when additionally a certain release trigger is provided (e.g. pH release). For the latter applications hydrophobic coating agents have been used as coatings made of these hydrophobic agents tend to be stable in the presence of water. Examples of hydrophobic coating agents that have been used in the manufacture of microcapsules include cellulosic ethers, zein, polylactide, OSA etc.
U.S. Pat. No. 3,821,422 describes a method of microencapsulating thiamine in a continuous ethylcellulose coating. The method described in the examples of the US patent involves dissolving ethylcellulose, polyethylene and thiamine in cyclohexane at 80° C., followed by cooling. As the temperature drops, solvated ethylcellulose develops as a separate phase due to the presence of polyethylene (polymer/polymer incompatibility). The solvated ethylcellulose distributed in the cyclohexane as droplets is said to tend to wet individual granules of thiamine and to envelop them. As the temperature drops further, the ethylcellulose looses solvent and develops in solid encapsulating walls. The microcapsules are said to protect the thiamine in the preparation of dough or batter, at elevated processing temperature and in the final product. Moreover, it is observed that the thiamine is released in the digestive juices of the gastrointestinal tract.
U.S. Pat. No. 4,462,982 describes ethylcellulose microcapsules wherein a polymer material which shows at least 1.2 times increase in weight by immersing it in water at 37° C. is incorporated into the ethylcellulose coating walls and/or the core material thereof. The examples of the US patent describe a process in which the microcapsules are prepared by dissolving ethylcellulose in cyclohexane, dispersing a core material in said solution, cooling the dispersion in the presence of the swellable polymer material until ethylcellulose separates out from the dispersion to form coating walls on and around the core material. The ethylcellulose microcapsules are said to protect the core material effectively and, when administered orally, release the core material rapidly in digestive organs such as the stomach.
GB-A 2 226 804 describes a process for the preparation of microcapsules comprising microencapsulating crystal granules of cyclohexane-insoluble active ingredients in a cyclohexane medium with ethylcellulose in the presence of an anionic surface-active agent or post-treating the drug granules microencapsulated with ethylcellulose by a cyclohexane-dissolved surface-active agent. The microcapsules described in the British patent application can be compressed directly into tablets without any further granulation. The microcapsules are said to provide a rapid release of the active ingredient contained therein.
U.S. Pat. No. 6,531,152 describes a delivery device for immediate localized release of a desired agent in the gastrointestinal tract, said device comprising:                a core comprising said agent, a core material that swells when exposed to an aqueous liquid, and a disintegrant;        a rigid coating surrounding said core that disintegrates and bursts when said core swells, said coating comprising water-insoluble hydrophilic particulate matter embedded in a water-insoluble carrier. The examples describe the preparation of coated tablets wherein a tablet core is spray coated with a coating suspension that was prepared by dissolving ethylcellulose in ethanol followed by the addition of calcium pectinate powder.        
An important drawback associated with the aforementioned methods for the preparation of microencapsulates resides in the use of organic solvents, notably cyclohexane. In order to allow microcapsules to be used in foodstuffs, beverages and pharmaceuticals, residual levels of organic solvents need to be reduced to very low levels. However, it is very difficult to achieve this and clearly it would be preferable to have no traces of organic solvents in these microcapsules at all.
WO 2008/056344 describes a method of producing microcapsules having a core and a coating encapsulating the core, the method comprising the steps of providing a core-forming fluid stream and a coating-forming fluid stream, providing a two spray nozzle arrangement having an outer nozzle disposed concentrically about a core nozzle, spraying the core-forming fluid stream from the core nozzle and the coat-forming fluid stream from the outer nozzle to produce microcapsules, and solidifying the microcapsules upon formation in a suitable gas. The Examples of the international patent application describes the preparation of microencapsulates by spraying the core-forming fluid stream and the coat-forming fluid stream in an atmosphere of nitrogen, using an inlet temperature of 100° C. Example 5 describes the preparation of a microencapsulate by spray drying a core-forming fluid stream consisting of a solution of sodium diclofenac and sodium fluorescein in ethanol together with a coat-forming fluid stream consisting of a solution of ethylcellulose in ethanol. Drawbacks of the process described in the international patent application are the explosion risks associated with the removal of organic solvents by spray drying and the difficulty of finding and maintaining adequate process conditions for the production of a high quality microencapsulate. Furthermore, due to the fact that the diameter of the core nozzle is typically in the range of 0.7-2 mm and that of the concentric nozzle in the range of 1.4-4 mm, the production of encapsulates having an average diameter in the range of 1-10 μm as disclosed in the examples of WO 2008/056344 is bound to yield encapsulates that will show significant leakage, especially at high payloads. Another problem that has been observed in relation to microcapsules that contain a hydrophobic coating is poor water dispersibility. Poor water dispersibility hampers the utilisation of such microcapsules in aqueous products (e.g. beverages) and may also adversely affect the intestinal release of the active components contained therein.
In addition, there is a need for microencapsulates that are very stable when used in water-containing foodstuffs, beverages, nutritional or pharmaceutical products, but that release their contents after ingestion, e.g. under the influence of the highly acidic conditions within the stomach.